Patent Document 1 below discloses a buffer device which includes a plurality of turn bars capable of winding a film-shaped material (a belt-shaped web) conveyed between two suction rollers. In the buffer device, each of the turn bars is movable up and down, a tension of the film-shaped material is detected by a sensor such as a load cell, and a movement amount of the turn bar is controlled by a feedback control based on this detection result, thereby inhibiting bending of the film-shaped material. Further, Patent Documents 2 to 4 below also disclose related arts.